Yes, any time on the 18th. Again the event will close as of 11:59 PM EST and winners will be announced on the 19th. Further details on exactly what will be happening will be announced tomorrow morning at 9:00 AM EST. There will be prizes awarded based first on number of items found and then on the time at which they were found, so, it is to your best advantage to be available early.

Edit: The event will close at 2am EST to allow the full day to be used. Results will take some time to compute depending on number of entries.